With modern telecommunications networks, a customer such as a business customer may select from an array of communication services aimed at providing cost-effective connections to geographically-dispersed sites maintained by the customer. The various available alternatives range, at one extreme, from ubiquitous direct dialing over the public network to, at the other extreme, specially provisioned private networks. Because of the vagaries of direct dialing, such as call blocking and connect-time cost, a customer with critical communication requirements most often selects the private network option. In particular, there has recently been a demand for large private data networks to connect, for example, numerous terminal devices such as reservation terminals to an arrangement of centrally located, fault-tolerant host computers so as to service consumer transactions. Data networks are generally implemented on private network facilities because, as exemplified above, computer applications necessitate continuous, on-line connections of terminals to centralized computers.
One common method of implementing a private line network is to interconnect various customer sites with DS1 digital facilities wherein numerous channels are multiplexed to generate a DS1 signal suitable for carriage over the high-rate links. A channel may have more than one DS0 (i.e. N.times.DS0, where 1.ltoreq.N.ltoreq.24). Each channel embedded in the DS1 will hereinafter be referred to as a fractional DS1 channel. At the destination end (far end), the high-rate signals are demultiplexed to recover the fractional DS1 channels. Oftentimes, the DS1 facilities are provided by a common carrier over carrier-owned digital facilities on a long-term or semi-permanent basis. DS1 facilities are provisioned by the common carrier through static cross-connect switches generally referred to as a Digital Access and Cross Connect System (DACCS). Unlike telephone-carrier switches which handle telephone call setups, a DACCS establishes routes which may be connected for years.
Efficient use of DS1 facilities requires channel grooming. Because of channel grooming, the fractional DS1 channels which are carried over each DS1 link may not all be derived from the same customer site. Specifically, the fractional DS1 channels that form an out-going DS1 signal from a DACCS are usually composed of fractional DS1 channels that originate from a plurality of in-coming DS1 facilities that come from different customer sites. Typically, facility failures in private networks require restoration efforts that are correspondingly sophisticated and time-consuming. Accordingly, facility engineers have sought techniques to provide for efficient and automated restoration of facilities when deleterious service conditions are detected.
Recently, a service has been introduced by one inter-exchange carrier which provides switched digital data service at fractional DS1 speeds. In using the service, a customer is able to establish a back up dial-up link to restore a failed private line fractional DS1 channel. However, such a service has the disadvantage of blocking, that is, a link between the end points is not always available. Moreover, it takes at least a few seconds to establish a connection over such a link each time a dial-up is attempted. In the period of a few seconds, a significant loss of data can occur.
A need exists in the art for a relatively simple technique for efficiently and automatically restoring telecommunications service over a fractional DS1 channel while providing essentially uninterrupted communications in the event that a DS1 facility, which carries that channel in a private line network, fails.